If Texas linebacker Derrick Johnson can take heart in anything, it's that scouts are always notorious misers with defensive evaluations.

They'll be the first to admit that finding a rookie who can score touchdowns is tricky, but finding one who can prevent them is brain surgery. Which is why guys like Johnson are so often nitpicked to death in the waning days before the draft. It's a brutal reality that has led to him sliding down draft boards, from a top-three selection in February to a top-seven selection in March to barely hanging on in the top 10 in April.

He's hardly the first. The same thing happened to Miami linebacker Jonathan Vilma a year ago. Despite leaving the Hurricanes as the best linebacker in college football, Vilma watched his resume get riddled with holes as the 2004 draft approached. According to one team scouting report, he lacked the "bulk" and "strength" to play middle linebacker, and he had "difficulty shedding blockers."

A few months after that report, Vilma, who was drafted by the New York Jets with the 12th overall pick, became the best rookie linebacker in the NFL and was being hailed as one of the league's brightest young stars. Even apparent brain surgeons lose sight of the big picture.

Tampa Bay Buccaneers coach Jon Gruden has taken that wide-screen view of Johnson's skills. "You watch [Texas'] defense on tape, and the guy is constantly in the picture," Gruden said. "He's always making things happen."

Ultimately, that's what the search for defensive prospects boils down to. The ones with the ability to shine, to make things happen, are the ones who churn out big rookie seasons like Vilma – not just starting, but starring.

That ability is what makes Johnson the leader of this season's batch of impact defenders. Below are the positions expected to produce at least one significant defensive rookie in the NFL this season, along with the likeliest player to shine.

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Derrick Johnson, Texas
Like Vilma last year, people are complaining about Johnson's ability to take on blockers. But as Gruden pointed out, Johnson has found a way to disrupt offenses for four years at a major program.
His speed and added bulk should suit him well at the outside linebacker spot in the NFL and make him an instant starter. Though he might not be able to put together a rookie season like Vilma did in 2004, he shouldn't be far off the numbers D.J. Williams put up in his debut with the Denver Broncos (114 total tackles, two sacks, one interception).

Florida's Channing Crowder is another linebacker likely to fill up the stat sheet. But I'd be wary about counting on major first-year production from guys being converted into the linebacker spot from other positions – like Maryland defensive end Shawne Merriman and Georgia safety Thomas Davis (if both do end up as linebackers in the NFL). Such transitions can be murder on a player's early production.